1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive element having an electrode layer provided on a substrate and a piezoelectric/electrostrictive layer provided on the electrode layer, and a manufacturing method of the element.
2. Description of the Related Art
In a piezoelectric/electrostrictive element, for example, an electrode layer containing a metal as a main component is secured onto the surface of a (ceramic) substrate containing zirconia as a main component, and a piezoelectric/electrostrictive layer is secured onto the electrode layer (see Patent Document 1).
Such a piezoelectric/electrostrictive element can be prepared, for example, by first forming, on the surface of the substrate, a coat layer as a first layer which can become the electrode layer, and forming, on this first layer, a second layer (which becomes the piezoelectric/electrostrictive layer) containing a piezoelectric/electrostrictive material, followed by heating and firing at a predetermined temperature and then cooling. The electrode layer itself may be formed as the first layer from the beginning, and in this case, the second layer is only fired in the step of the firing. It is to be noted that documents of the prior art are as follows:    [Patent Document 1] JP-A-2001-284677    [Patent Document 2] JP-A-2006-165007    [Patent Document 3] WO2006/109501A1
Moreover, in such a piezoelectric/electrostrictive element, for example, a through channel is provided on the side of the substrate opposite to the electrode layer and the piezoelectric/electrostrictive layer, and the element can function as a sensor of a fluid which passes through this through channel.
For example, the piezoelectric/electrostrictive element may be resonated, and by the scant change of a resonance frequency, the presence of the fluid can be detected. In this case, by use of the presence of a correlation between the amplitude of the piezoelectric/electrostrictive element and the viscous resistance of the fluid which comes in contact with the piezoelectric/electrostrictive element, properties such as the density, concentration and viscosity of the fluid can be measured.
Specifically, when this piezoelectric/electrostrictive element is vibrated in the presence of the fluid, the piezoelectric/electrostrictive element meets with a mechanical resistance owing to the viscosity of the fluid. Here, the mechanical vibration state of the piezoelectric/electrostrictive element can be replaced with an equivalent electric circuit. Therefore, the electric constant of the equivalent circuit of a piezoelectric/electrostrictive film of the piezoelectric/electrostrictive element changes based on the viscous resistance exerted on the piezoelectric/electrostrictive element. This change of the electric constant is detected to measure properties such as the viscosity, density and concentration of the fluid.
Examples of the fluid whose properties can be measured by the piezoelectric/electrostrictive element include both a liquid and a gas. Examples of the liquid as a measurement target include not only a liquid comprising a single-component liquid containing an only main medium selected from the group consisting of water, alcohol and oil but also a liquid (a slurry, a paste or the like) obtained by adding (dissolving, mixing, dispersing or suspending), to such a main medium, another soluble, sparingly soluble or insoluble medium.
Moreover, examples of the above electric constant include loss coefficient, phase, resistance, reactance, conductance, susceptance, inductance and capacitance. In particular, the loss coefficient or phase having one maximum or minimum change point in the vicinity of the resonance frequency of the equivalent circuit is preferably used. In consequence, not only the viscosity but also the density or concentration of the fluid can be measured. For example, the concentration of sulfuric acid in an aqueous sulfuric acid solution can be measured. It is to be noted that as an index for detecting the change of the vibration state, the change of the resonance frequency may be used instead of the above electric constant, if any special problem does not occur from the viewpoint of measurement accuracy or durability.
However, in this type of piezoelectric/electrostrictive element, if the characteristics (sensor sensitivity, etc.) of the piezoelectric/electrostrictive element are improved by further increasing a flexural displacement amount or an internal stress value, a problem of durability, for example, cracking in the piezoelectric/electrostrictive layer or the like occurs. In this case, the output or resonance frequency of the piezoelectric/electrostrictive element noticeably changes beyond a sensing range, and hence the element cannot be used as the sensor.